1. Field of the Invention
The present invention relates to a signal encoding apparatus, a signal encoding method, a signal decoding apparatus, a signal decoding method, programs and recording mediums that can suitably be used for expanding a sub-band signal limited to a certain frequency band at the encoding side to a broader frequency band at the decoding side.
2. Description of the Related Art
It has been made technically possible in recent years to compress the data of a CD (compact disc) and reduce the quantity of data to about 1/10 of the original quantity by utilizing the mechanism of the auditory sense of human being for the purpose of highly efficiently encoding audio signals. Currently, products produced by utilizing such techniques are marketed and compressed audio data are recorded on smaller recording mediums and distributed by way of networks.
With high efficiency compression coding, many recording companies have developed their own formats to make it possible to freely control the sound quality and the bit rate to a certain extent at the encoding side within the limits defined by the format. For instance, two modes of LP2 and LP4 that utilize same high efficiency compression techniques are available as long recording modes for MiniDiscs (MDs) (trademark, available from Sony Corporation). In the LP4 mode, data are compressed to a half of the data of the LP2 mode to realize recording hours twice as many as the recording hours of the LP2 mode, although the sound quality is lower.
However, with such high efficiency compression techniques, the bit rate and the sound quality are the clear target for design and standardization. In other words, the sound quality is remarkably degraded when the bit rate is lowered without modifying the standard (format). To avoid such a problem, techniques for improving the high efficiency encoding algorithm at the encoding side and those for limiting high frequency signals in a frequency band where the auditory sense of human being is not keen and assigning the surplus bits to low frequency signals are generally utilized.    [Non-Patent Document 1] Information Technology—Coding of Audio-Visual Objects—Part 3: Audio, ISO/IEC 14496-3: 2001